1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, a device and an optical modulation element, and more particularly to a technique which generates a crystallized semiconductor film by irradiating a non-single-crystal substance such as a non-single-crystal semiconductor film with a laser light having a predetermined light intensity distribution.
2. Description of the Related Art
A thin film transistor (TFT) used for a switching element or the like which selects a display pixel in, e.g., a liquid crystal display (LCD) has been conventionally formed by using amorphous silicon or polysilicon.
Polysilicon has a higher mobility of electrons or holes than that of amorphous silicon. Therefore, when a transistor is formed by using polysilicon, a switching speed and hence a display response speed become higher than those in case of forming the same by using amorphous silicon. Further, a peripheral LSI can comprise a thin film transistor. Furthermore, there is an advantage of reducing a design margin of any other component. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at a higher speed.
Since polysilicon comprises an aggregation of crystal grains, when, e.g., a TFT transistor is formed in this polysilicon, crystal grain boundaries present in a channel region, this crystal grain boundary serves as a barrier, and a mobility of electrons or holes is reduced as compared with that of single-crystal silicon. Additionally, each of many thin film transistors formed by using polysilicon has a different number of crystal grain boundaries formed in a channel region, and this difference becomes irregularities, resulting in a problem of unevenness in display in case of a liquid crystal display. Thus, there has been recently proposed a crystallization method which generates crystallized silicon having a crystal grain with a large particle size enabling at least one channel region to be formed in order to improve the mobility of electrons or holes and reduce irregularities in number of crystal grain boundaries in a channel portion.
As this type of crystallization method, there has been conventionally known a “phase control ELA (Excimer Laser Annealing) method” which generates a crystallized semiconductor film by irradiating a phase shifter approximated in parallel with a polycrystal semiconductor film or a non-single-crystal semiconductor film with an excimer laser light. The detail of the phase control ELA method is disclosed in, e.g., Journal of The Surface Science Society of Japan, Vol. 21, No. 5, pp. 278-287, 2000.
In the phase control ELA method, a light intensity distribution having an inverse peak pattern (a pattern in which a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery (lateral sides)) in which a light intensity at a point corresponding to a phase shift portion of a phase shifter is lower than that in the periphery is generated, and a non-single-crystal semiconductor film (a polycrystal semiconductor film or an amorphous semiconductor film) is irradiated with a light having this light intensity distribution with an inverse peak shape. As a result, a temperature gradient is generated in a fusion area in accordance with a light intensity distribution in an irradiation target area, a crystal nucleus is formed at a part which is solidified first or a part which is not molten in accordance with a point where the light intensity is minimum, and a crystal grows from the crystal nucleus in a lateral direction toward the periphery (which will be referred to as a “lateral growth” or a “growth in the lateral direction” hereinafter), thereby generating a single-crystal grain with a large particle size.
Further, “Growth of Large Si Grains at Room Temperature by Phase-Modulated Excimer-Laser Annealing Method” by H. Ogawa et al., IDW′ 03 Proceedings of the 10th International Display Workshops, p. 323 releases a crystallization method which generates a crystal grain by irradiating a non-single-crystal semiconductor film with a light having a V-shaped light intensity distribution which can be obtained through a phase shifter and an image formation optical system. Furthermore, this known reference discloses that it is desirable for an intensity distribution of a light with which the non-single-crystal semiconductor film is irradiated to vary in a V-shape in an intensity range of 0.5 to 1.0 when the maximum value of the intensity is standardized as 1.0.
In the crystallization method disclosed in this known reference, a pulse oscillation type laser light source like an excimer laser light source is used, and each typical pulse light emission time thereof is 20 to 30 nsec (nanoseconds). This time is set in order to obtain a large light intensity required to melt a semiconductor by concentrating an light emission energy on a part of the semiconductor like silicon in a very short time. As a result, a semiconductor can be irradiated with the same light intensity distribution (V-shaped) for each pulse light emission time.
Disadvantages caused due to irradiating a semiconductor with the same V-shaped light intensity distribution in the prior art disclosed in the last known reference will now be described hereinafter with reference to FIGS. 18A to 18D (FIGS. 18C and 18D are views showing calculation results concerning a change in temperature distribution in an a-Si (noncrystalline silicon or amorphous silicon) layer obtained when the a-Si layer is irradiated with a light beam having a V-shaped light intensity distribution over a fixed time in accordance with the prior art). On the occasion of calculating this temperature distribution, a calculation method described in “A New Nucleation-Site-Control Excimer-Laser-Crystallization Method” by Mitsuru Nakata et al., Jpn. J. Apple. Phys. Vol. 40 (2001) Pt. 1, No. 5A, 3049p is adopted, and this cited reference is incorporated herein as a reference. Moreover, on the occasion of calculating a temperature distribution, an influence of latent heat which is absorbed/generated when a-Si is molten/solidified is ignored. As calculation conditions, there is assumed a layer structure comprising an SiO2 layer having a thickness of 200 nm, an a-Si layer having a thickness of 200 nm, and an SiO2 layer having an infinite thickness in the order from a light incidence side. It is assumed that a maximum light intensity is 1.0×1011 W/cm2 in a unit light intensity distribution having a V-shape shown in FIG. 18A (which indicates one mountain-shape unit intensity portion in a light intensity distribution comprising a plurality of V-shaped unit light intensity distributions which are continuously formed in the drawing), and each pulse light emission time is 20 nsec as shown in FIG. 18B. Additionally, it is assumed that a-Si has a thermal conductivity of 24 W/mK, specific heat of 861 J/KKg and a density of 2340 Kg/m3. Further, it is assumed that SiO2 has a heat conductivity of 1.5 W/mK, specific heat of 1000 J/KKg and a density of 2300 Kg/m3.
Referring to FIG. 18C showing a change in temperature distribution during pulse light emission, it can be understood that the temperature distribution keeps the V-shape (which indicates a chevron part in the continuously formed V-shaped light intensity distribution in the drawing) and achieves an increase in temperature with an elapse of time in 20 nsec during which a light beam having a V-shaped light intensity distribution is applied. However, referring to FIG. 18D showing a change in temperature distribution after pulse light emission, it can be recognized that a temperature is gradually reduced with an elapse of time after end of pulse light emission and a temperature gradient in a high-temperature region (a peak portion) in the V-shaped temperature distribution is flattened with a time. A factor of this phenomenon is thermal diffusion in an in-plane direction in the a-Si layer.
FIG. 19 is a view schematically showing an advancing state of crystallization of a-Si involved by a change in temperature distribution depicted in FIG. 18C. In crystallization of Si involved by a change in temperature distribution depicted in FIG. 18C, as shown in FIG. 19, after an entire light reception region of a-Si is once molten and incidence of a laser light is interrupted, partial crystallization occurs at a part where a temperature is lowest, i.e., a bottom part of the V-shaped temperature distribution (and hence a light intensity distribution). Thereafter, a crystal grows in the lateral direction with this crystallized part serving as a nucleus due to heat of a temperature gradient in the V-shaped temperature distribution. However, when the crystal growth is in the final stage and a high-temperature region (in the vicinity of a peak) of the V-shaped temperature distribution (and hence the light intensity distribution) is reached, the temperature gradient in the high-temperature region is in a flat state (a state in which the temperature distribution is rounded) due to an advance of thermal diffusion.
Therefore, the primarily desired crystal growth is terminated before reaching the high-temperature region, an undesired crystal nucleus is generated in the high-temperature region in the V-shaped temperature distribution, and this high-temperature region is polycrystallized. As a result, the influence of thermal diffusion in the final stage of the crystal growth disables realization of the sufficient crystal growth from the crystal nucleus and hence generation of a crystallized semiconductor having a crystal grain with a large particle size. In this case, the “crystal grain with a large particle size” means a crystal grain having a size with which a channel region of one TFT can be completely formed in the crystal grain. Further, in this case, a margin of a positioning accuracy is narrowed, for example.
Although the influence of latent heat is not considered in the above-described calculation, a temperature increases in the vicinity of a solid-liquid interface due to latent heat generated during solidification. This phenomenon is introduced in “formation of an Si thin film with a huge crystal grain using an excimer laser” by Masakiyo Matsumura, Journal of The Surface Science Society of Japan, Vol. 21, No. 5, pp. 278, 2000. Analogizing from a result introduced in this reference, it can be conjectured that a temperature distribution is affected by latent heat and is as shown in FIG. 20 when a temperature gradient in a high-temperature region is flattened in FIG. 19. In this case, it can be considered that flattening of the temperature gradient due to the influence of emission of latent heat further widely occurs and the crystal growth from the crystal nucleus from which crystallization has first started becomes shorter (a crystal grain which is short in the lateral direction is obtained).